The present invention relates generally to exponential converter circuitry, and, more particularly, to a temperature-independent exponential converter capable of generating a temperature-independent signal which is exponentially related to an input signal applied thereto.
Many types of circuitry utilize exponential circuitry to generate a signal which is exponentially related to an input signal applied thereto. For instance, circuitry forming portions of components of a communication system constitutes one such type of circuitry which advantageously utilizes such exponential circuitry. Typically, when exponential circuitry forms portions of such communication components, the exponential circuitry is utilized to convert linear-scaled signals into decibel-scaled signals. (A decibel is a value related to an exponential value.)
A transmitter and a receiver comprise the component portions of a communication system. The transmitter and the receiver are interconnected by a transmission channel, and an information signal is transmitted by the transmitter upon the transmission channel to the receiver which receives the transmitted, information signal.
A radio communication system comprises a communication system wherein the transmission channel is formed of a radio-frequency communication channel. The radio-frequency communication channel is defined by a range of frequencies of the electromagnetic frequency spectrum. To transmit an information signal upon the radio-frequency communication channel, the information signal must be converted into a form suitable for transmission thereof upon the radio-frequency channel.
Conversion of the information signal into a form suitable for transmission thereof upon the radio-frequency communication channel is accomplished by a process referred to as modulation wherein the information signal is impressed upon a radio-frequency electromagnetic wave. The radio-frequency electromagnetic wave is of a value within a range of frequencies of the frequencies which define the radio-frequency communication channel. The radio-frequency electromagnetic wave upon which the information signal is impressed is commonly referred to as a "carrier signal", and the radio-frequency electromagnetic wave, once modulated by the information signal, is referred to as a modulated signal.
The information content of the modulated signal occupies a range of frequencies, sometimes referred to as the modulation spectrum. The range of frequencies which comprise the modulation spectrum include the frequency of the carrier signal. Because the modulated signal may be transmitted through free space upon the radio-frequency channel to transmit thereby the information signal between the transmitter and the receiver of the radio communication system, the transmitter and the receiver portions of the communication system need not be positioned in close proximity with one another. As a result, radio communication systems are widely utilized to effectuate communication between a transmitter and a remotely-positioned receiver.
Various modulation techniques have been developed to modulate the information signal upon the carrier signal to form the modulated signal, thereby to permit the transmission of the information signal between the transmitter and the receiver of the radio communication system. Such modulation techniques include, for example, amplitude modulation (AM), frequency modulation (FM), phase modulation (PM), frequency-shift keying modulation (FSK), phase-shift keying modulation (PSK), and continuous phase modulation (CPM). One type of continuous phase modulation is quadrature amplitude modulation (QAM).
The receiver of the radio communication system which receives the modulated signal contains circuitry to detect, or to recreate otherwise, the information signal modulated upon the carrier signal. The circuitry of the receiver typically includes circuitry to convert downward in frequency the modulated signal received by the receiver in addition to the circuitry required to detect the information signal. The process of detecting or recreating the information signal from the modulated signal is referred to as demodulation, and such circuitry for performing the demodulation is referred to as demodulation circuitry.
In some receiver constructions, circuitry including a processor (referred to as a digital signal processor or a DSP) is substituted for conventional demodulation circuitry.
The signal actually received by the receiver of a radio communication system frequently varies in magnitude as a result of reflection of the transmitted signal prior to reception by the receiver. Typically, the signal actually received by the receiver is the summation of the transmitted signal which travels along a plurality of different paths forming signal paths of differing path lengths. Because the transmission channel upon which the modulated signal is transmitted typically includes a plurality of different signal paths, a transmission channel is frequently referred to as a multi-path channel. Transmission of the signal upon signal paths of path lengths greater than the path length of a direct path results in signal delay as the summation of the transmitted signal upon the multi-path channel is actually a summation of signal transmitted by a transmitter and received by the receiver at different points in time.
Such signal delay results in interference referred to as Rayleigh fading and intersymbol interference. Such interference causes signal amplitude variance of the signal received by the receiver. When the communication system, formed of a transmitter and receiver, comprises a transmitter and receiver of a mobile communication system (such as a cellular telephone system), when a receiver is positioned in a vehicle traveling at 60 MPH, the signal strength of a modulated signal transmitted by the transmitter, and actually received by the receiver, may vary by approximately 20 decibels during a five millisecond period.
Gain control circuitry oftentimes forms a portion of the receiver circuitry alternately to amplify the received signal and limit the magnitude of the received signal to overcome the effects of such fading.
Gain control circuitry typically utilizes signals which are scaled in terms of decibels per volt. As a decibel is a logarithmic value, exponential conversion circuitry also typically forms a portion of the gain control circuitry of the receiver circuitry.
Existing exponential conversion circuitry is available which is operative to form an exponential output signal responsive to application of a linear input signal thereto.
For instance, disclosed in a text entitled, "IC Op-Amp Cookbook," by Howard W. Sams, copyright 1974, pages 214-216 is an antilog generator for forming an exponential signal responsive to application of a signal thereto. The antilog generator is comprised of discrete components.
Also, an integrated circuit, INTERSIL Part No. ICL8049, discloses a similar such structure in integrated circuit form. Additionally, an integrated circuit, INTERSIL Part No. ICL8048, discloses a logarithmic converter for performing a logarithmic conversion.
The existing circuitry for generating an exponential signal responsive to application of an input signal thereto forms an exponential signal which is temperature-dependent. The actual signal generated by such circuitry is therefore temperature-dependent, viz., the actual, exponential signals generated by such circuitry are of values which vary corresponding to the temperature of the circuitry. Therefore, the signals generated by such existing circuitry are not dependent solely upon the values of the signals supplied thereto, but also upon temperature.
While both the antilog generator and the integrated circuit equivalents thereof attempt to provide temperature-compensation to minimize the dependence of the signal formed by the circuitry upon temperature, such attempts may not totally cancel the temperature-dependency of the signal.
The antilog generator disclosed by Sams includes a discrete thermistor. As the temperature of the thermistor is not necessarily equal to that of the amplifier of the antilog generator, the attempt to compensate for the temperature-dependency of the signal is frequently inadequate.
The antilog generator disposed upon the integrated circuit attempts to compensate for the temperature-dependency of the signal generated therefrom by forming the integrated circuit by a hybrid production process. An integrated circuit formed of a hybrid production process is of at least two different types of materials. Such a process increases production costs as well as material costs, and, in any event, the temperature-compensation circuitry of such integrated circuits again may not totally cancel the temperature-dependency. The attempt to compensate for the temperature-dependency in this manner is, therefore, frequently inadequate.
Accordingly, gain control circuitry of receiver components of a radio communication system which utilizes such conventional exponential conversion circuitry generates signals which vary corresponding to the temperature level of the circuitry. Therefore, gain control signals generated by such gain control circuitry are, at least in part, variable responsive to temperature levels. As such temperature dependency adversely affects the functioning of the receiver gain control circuitry, the resultant gain control of a received signal is subject to error.
What is needed, therefore, is exponential conversion circuitry which generates an exponential signal which is temperature-independent.